With digitization in broadcasting or mobile radio communication, a digital modulation system has been developed in recent years. In particular, in the mobile radio communication, orthogonal frequency division multiplex (hereinafter referred to as OFDM) modulation, which is durable against multipass interference, has been under examination for adoption. OFDM modulation is a system for distributing transmitted digital data on multiple carriers (hereinafter referred to as subcarriers) which are mutually orthogonal and for modulating the digital data transmitted on each carrier. OFDM modulation is advantageous in that the frequency utilization factor is high, it hardly supplies disturbance to other digital data transmitted on surrounding carriers, and it is virtually free from the effect of multiple path interference.
FIG. 1 is a block diagram showing a conventional OFDM modulator/demodulator.
Transmitted data such as a QPSK modulated or QAM modulated signal is input through an input terminal 1. The transmitted data is supplied to a serial/parallel converter 3 of an OFDM modulator 2, where the data is converted into low speed parallel data comprising multiple symbols. The number of symbols per parallel data coincides with the number of sub-carriers. An inverse fast Fourier transform (hereinafter referred to as IFFT) circuit 4 modulates between several hundred and several thousand mutually orthogonal sub-carriers. The number of sub-carriers is set according to a number of using points of the IFFT circuit 4. The transmitted data which has been OFDM modulated by the IFFT circuit 4 is supplied to a parallel/serial converter 5 where it is converted into serial data and supplied to a guard period adding circuit 6. The guard period adding circuit 6 adds a guard period to the serial data in order to prevent the multiple path interference and outputs the data to a transmission line (not shown).
FIG. 2 is a typical waveform diagram showing the transmitted data along with a guard period.
As the transmitted data is modulated after distributed into several hundreds or thousands of sub-carriers in the OFDM modulation system, the modulation symbol rate of sub-carriers becomes extremely low and the period per symbol become extremely long. Consequently, the transmitted data is hardly subject to delays caused by reflecting waves. Further, the effect of multiple path interference can be eliminated effectively by placing a guard period in front of the available symbol period. A guard period adding circuit 6 provides a guard period which replicates a latter half of a corresponding available symbol period as shown in FIG. 2. If the delay time of multiple path interference is within the guard period, it is possible to prevent inter-symbol interference resulting from delayed adjacent symbols by limiting demodulation of the available symbol period signal to the time of demodulation.
In the OFDM demodulation circuit 7, data received from a transmission line (not shown) is supplied to a guard period removing circuit 8. The guard period removing circuit 8 extracts signals in the available symbol period from the received data, and supplies the extracted signals to a serial/parallel converter 9. The serial/parallel converter 9 converts serial data into parallel data for every sub-carrier, and outputs the converted parallel data to a fast Fourier transform (hereinafter referred to a FFT) circuit 10. The FFT circuit 10 demodulates sub-carriers through the FFT operation. The demodulated signal output from FFT circuit 10 is converted into serial data by a parallel/serial converter 11 and is output as received data.
In order for FFT circuit 10 to execute accurate demodulation, it is necessary to obtain a timing synchronization (hereinafter referred to as the symbol synchronization) of the available symbol period. Therefore, as transmission data is transmitted after orthogonal modulation, a carrier synchronization must be obtained at a receiver section for the proper orthogonal demodulation. Because the OFDM modulated wave shown in FIG. 2 is a waveform similar to random noise, it is difficult to obtain symbol synchronization and carrier synchronization based on the OFDM modulated wave.
Therefore, conventional OFDM synchronization demodulation circuit add a separate reference signal to attain symbol synchronization (see CCIR Rec. 774). FIG. 3 is an explanatory diagram for explaining such a conventional symbol synchronization method.
As described above, a guard period has been added to the transmitted data. That is, as shown in FIG. 3, transmitted data corresponding to each symbol includes both available symbol period S and guard period G. Further, a non-signal period (hereinafter referred to as the null symbol period) for symbol synchronization is added for every several tens of symbol periods. By detecting the null symbol period contained in transmitted data, it is possible to obtain the symbol synchronization at the demodulator section. That is, by detecting a demarcation timing between the null symbol period and the guard period from a modulated wave envelope, the available symbol period timing is obtained.
FIG. 4 is a graph for explaining a carrier synchronization method of another conventional OFDM synchronization demodulation circuit which has been described in "Summary of OFDM Experiments done by the ARTC". In FIG. 4, frequency is plotted on the X-axis and amplitude of a spectrum plotted on the Y-axis. The central frequency band of FIG. 4 indicates sub-carriers modulated by transmitted data. Sub-carriers at both sides of the frequency band are not modulated, but are instead used as pilot carriers 15 and 16. At the demodulator section, carrier synchronization is attained by detecting the pilot carriers.
However, a method for executing symbol synchronization based on cyclically transmitted null symbols may be inaccurate because null symbols may be disturbed and erroneously detected. When null periods go undetected or if they are spaced too far apart, the normal demodulating operation is not carried out for a long time, until next null symbol was detected in this case. If null symbols are sent frequently to solve the problem, transmission efficiency drops. Further, in a method for executing the carrier synchronization using pilot carriers, carrier synchronization cannot be attained if pilot carriers are disturbed.
In case of a conventional OFDM synchronization demodulation circuit as described above, the normal demodulation operation is inhibited since symbol synchronization is not achieved when a null symbol added to transmitted data is disturbed. In addition, no carrier synchronization is attained if pilot carriers are disturbed.